The last two columns are used to store binary data, these columns will always have values
but not selected/accessed or not used in where clause.

I query the table either by txnid or orgtxnid(indexed) only.

Is there anything I can do to increase the performance (fast search)?
Shouldn't I store binary data in varchar2?
Should I use LOB instead of varchar2?
Should I split the table into two.(I am afraid of two inserts)

Not sure where that wiki entry came from, but I'm pretty sure that's not 4NF. There is a pretty detailed description on Wikipedia that describes 4NF. The one similarity with this discussion is that both involve the separation into multiple tables.

As for the OPs problem, if most access is by those two keys, and those two keys are highly selective, then nothing is to be gained from splitting the table. This would only benefit long range scans (assuming not all queried columns are in the index) and full table scans.Ross Leishman